The present invention relates to a packet communication system in which voice information, picture information, high-speed data information or the like is communicated in the form of packets, and in particular to a transmission method and apparatus for transmitting the above described information. Especially the present invention relates to a packet communication method and apparatus suitable for packet transmission in which the above described information or the like is transmitted as packets in an asynchronous transfer mode (ATM).
In the field of communication, the ATM method recently attracts attention as a transmission technique capable of handling traffics ranging from a low speed to a high speed in a one-dimensional manner. In the ATM method, various kinds of information is converted into a packet form and transmitted in an asynchronous transfer mode as described above. All information transmitted from a user inclusive of signals is divided into short, fixed-length packets referred to as cells. Fixed-length cells are suitable for higher speed and parallel processing. Each cell includes a header portion having a capacity of several bytes and an information portion having a capacity of several tens to several hundreds bytes which will be described later. The header portion has therein a logical channel number for identifying the destination of transmission (transmission destination communication node: packet communication apparatus) or the call (transmission source subscriber terminal) and an error check code for detecting a header error, for example.
In such an ATM method, all information is handled with fixed-length cells in a unified manner and an arbitrary speed including a variable speed can be coped with. Therefore, the ATM method can cope flexibly with uncertain diverse demand and cope with communication of various media ranging from a low-speed medium to a high-speed medium. That is to say, the ATM method also has a possibility of a multi-media integrated network.
Further, an apparatus in which cells are transmitted does not have slots fixed to respective calls. Therefore, cells can be transmitted to arbitrary positions, resulting in efficient multiplexing.
This multiplexing effect is exhibited in a paper written by Noguchi etc. ("A method for prescribing the statistical multiplexing effect and burst traffic in ATM switching network", IEICE Trans., Vol. J73B, No. 1, PP. 25-33, January 1990). FIG. 3 (P. 27) of this paper shows characteristic curves of the maximum multiplexing factor Nmax capable of satisfying the quality standards condition of the peak value (maximum value) of the information generation speed with respect to the transmission path capacity ratio Rm. It is indicated that a large multiplexing effect is obtained in a region where the maximum speed per call (peak value of information generation speed) is smaller than the transmission path capacity.
If the maximum speed per call approaches the transmission path capacity, i.e., in a region where the maximum speed is larger than the transmission path capacity, however, the multiplexing effect is lowered, resulting in a problem.
In the prior art, no attention is paid to this point.